A cyberattack similar to previous hacker intrusions from China penetrated computer networks for months at USIS, the government's leading security clearance contractor, before the company noticed, officials and others familiar with an FBI investigation and related official inquiries told The Associated Press.

USIS reported the cyberattack to federal authorities on June 5, more than two months before acknowledging it publicly. The attack had hallmarks similar to past intrusions by Chinese hackers, according to people familiar with the investigation. Last March, hackers traced to China were reported to have penetrated computers at the Office of Personnel Management, the federal agency that oversees most background investigations of government workers and has contracted extensively with USIS.